The application claims priority of Canadian Patent entitled xe2x80x9cMethod for Producing Casting Moldsxe2x80x9d having application serial number 2,356,631 and filed on Aug. 31, 2001.
The invention relates to the production of custom-made casting molds. More specifically, the present invention is concerned with a method used to cast molds for dental prostheses and other such molds.
The conventional approach to produce custom casting molds has traditionally been the lost wax casting process. This process is based on the following principles:
1. A model or pattern of the desired finished product is made in wax;
2. This pattern is mounted with sprues and reservoirs to support the pattern in the desired casting position, provide passageways for wax elimination and form lines through which molten metal enters the mold;
3. This pattern is surrounded by a creamy investment plaster that hardens to form a mold;
4. This mold is heated to melt away the wax pattern that is xe2x80x9clostxe2x80x9d;
5. Metal is then cast into the cavity left by the xe2x80x9clost waxxe2x80x9d, thus duplicating the original wax pattern (the quantity of metal is calculated based on measurements of either the weight or the volume of the wax pattern);
6. The mold is then destroyed to recover the casting.
This process dates back in time beyond Egypt""s pyramids. Examples of castings include the Shang Dynasty in China, 16th century Europe, and the Aztecs of Mexico, to name a few. These are monuments to the abilities of casters down through the ages. Today, the process is becoming even more popular because of the need for a near-finished casting.
Refinements of this process were developed by a dentist in 1907, and applied to the casting of gold inlays and dental bridgework. Today""s craftsmen apply the very same techniques in making parts for models, fishing lures, specialised machine components and endless pieces of jewellery.
Since every detail created in the surface of the finished wax model will ultimately be re-created in metal when the casting is complete, special care taken in making the wax pattern can minimise the finishing and polishing steps later.
To produce dental prostheses, the dental laboratory technician typically duplicates the patient teeth in plaster from an impression provided by a dentist, then uses the lost wax method to realise a casting mold, then invests the mold in the casting oven filling it with metal in fusion to build the substructure components of dental prostheses, then porcelain is applied and fused on the metal substructure to complete the prosthetic work. The casting oven either uses the centrifuge effect principle or a vacuum chamber.
The lost-wax method to realise a casting mold therefore generally consists of the following steps:
1. Apply die spacer on the surface of the plaster die to emulate the cement space;
2. Realise the prosthesis out of wax (by dipping and wax-up
3. technique);
4. Add wax pouring sprue (off-the-shelf item) on top of the wax prosthesis;
5. Add several prosthetic elements with pouring sprues on top of an investment tree (off-the-shelf element or not) with an adequate casting reservoir;
6. Add wax cooling sprues and vent sprues (off-the-shelf items) when desirable;
7. Install the wax ensemble in an investment cylinder;
8. Invest the ensemble with a refractory material (heating will eliminate the wax and result in a negative mold of the ensemble: casting reservoir, tree, sprues, prosthesis).
However, the lost-wax method is labour-intensive and also requires dexterity to manipulate this fragile wax structure without breaking it, twisting it or otherwise distorting it. Furthermore, the use of off-the-shelf items constrains the design of the casting mold.
The computer-aided design and manufacturing of the prosthesis is an alternative method for producing dental prosthesis. This method is typically used in conjunction with computer-controlled milling machines. Blanks are milled into prostheses. However, with this process, a large proportion of the blank material is rejected. When the prosthesis substructure is composed of a precious alloy, the scrap metal represents more value than the prosthesis itself.
Accordingly, an object of the present invention is to provide an improved wax casting mold and method for making same.
According to a first broad aspect of the present invention, there is provided a method for producing a dental prosthesis comprising: obtaining three-dimensional digital data relating to a patient""s dentition; designing a virtual prosthesis for the dentition using the three-dimensional digital data; transmitting digital data corresponding to the virtual prosthesis to an automated prototyping system; producing a prototype of the dental prosthesis with the automated prototyping system, the prototype made of a material that can be ablated; covering at least the prototype with a hardening material and removing the prototype from within said hardening material to produce a mold for the dental prosthesis; casting the dental prosthesis by filling the mold with a metal and removing the hardening material.
Preferably, the method also comprises designing virtual conduits and reservoirs to be produced with the prototype of the dental prosthesis and used in casting of the dental prosthesis. Virtual conduits and reservoirs are positioned on the prosthesis by simulation to minimise a quantity of metal needed to produce the prosthesis while improving an invested metal flow in order to fill completely the prosthesis during the casting.
Also preferably, the method comprises designing a virtual casting mold container, wherein the virtual conduits and reservoirs are configured on the virtual casting mold container to support multiple patterns in a desired casting position, provide passageways for fluid elimination, and form lines through which molten metal enters the mold.
Throughout this application, the term investment is used as a common term in the art of casting wherein investment casting designates a casting technique that uses destructible models covered in a refractory material and forming a jointless mold. The term investment casting can also refer to the lost-wax process. The process comprises pouring a metal into a mold obtained by coating a wax, plastic, or mercury gel prototype with a refractory material and removing the material using heat. The term refractory is used to mean a material or product other than metal or alloy with a pyroscopic resistance at least equal to 1500 degrees Celsius.